Laminates comprising one or more layers of polyimide and one or more layers of substrate materials such as metals and alloys may be used for a variety of applications. These applications include structures which provide both structural integrity as well as electrical circuitry such as in circuit boards or the like, and many other uses. When laminates are to be used for current carrying applications, such as circuit boards or the like, the metal substrates are selected because of their mechanical properties, as well as their electrical and/or magnetic properties. At least one of the metal substrates typically is copper or a copper alloy because of the high conductivity of copper and its alloys. The polyimide layer or layers are selected because of their dielectric properties.
There are many prior art processes for laminating polyimides to various types of metal sheets or foils. U.S. Pat. No. 4,675,246 to Kundinger, et al. describes various different types of laminate configurations used in different types of polyimides and different metal substrates. However, this patent discloses only conventional curing techniques for curing the polyimide materials. Additionally, U.S. Pat. No. 3,607,387 to Lanza, et al. discloses a thermoplastic polyimide used in contact with a wire conductor such that the circumference is encased. The thermoplastic polyimide is then encased with an intractable polyimide. This reference does not teach a process for lamination of sheets, only a process for dip coating or extrusion coating of wire.
U.S. Pat. No. 4,411,952 to Sasaki et al. describes the use of specific polyimides formed from reaction of 3, 3', 4, 4'- or 2, 3, 3', 4'- biphenyl tetracarboxylic acids having any one of a variety of diamine bridging units. These precursors are coated and then fully imidized to form a sheet which imidized sheet is then laminated to a conductor foil by conventional heat/pressure processes.
U.S. Pat. No. 4,503,285 to Darms, et al. discloses the use of polyamide/polyamide acid copolymers and block copolymers as useful precursor solutions for obtaining polyamide/polyimide composite films with high adhesion to metal foils. The polyamide/polyamide acids are coated onto a conducting foil and cured in situ. The foil is then photopatterned and etched. The curing is conventional and forms a polyamide/polyimide film.
U.S. Pat. No. 4,543,295 to St. Clair et al. discloses the use of fully imidized thermoplastic polyimide films placed between metal foils, and the use of thermoplastic polyamic acid, applied to the fully imidized polyimides or metal foils to act as a thermoplastic adhesive. While this patent discloses the use of linear aromatic polyamic acids and polyimides as adhesives, it stresses only the importance of adequate thermal treatments to minimize outgassing during lamination processing. Apparently there is no attempt to minimize thermally initiated interchain crosslinkage or polyimide densification during either polyimide curing or lamination processing.
U.S. Pat. No. 4,681,654 to Clementi, et al. discloses a method for making a polyimide based chip carrier by application of either a polyamic acid or, a fully imidized polyimide solution onto a metal carrier. The polyimide film is then cured by either thermally induced imidization in the case of the polyamic acid film, or by simple solvent evaporation in the case of the preimidized film, so that the metal carrier can be removed to allow a thin free-standing polyimide film to be formed. This free standing polyimide film can be bonded to a support frame or roll carrier for further processing with the use of an adhesive. This adhesive is comprised of either a polyimide, acrylic, or epoxy resin.
U.S. Pat. No. 4,883,718 to Ohta, et al. teaches the use of specific classes of polyimides as thermoplastic adhesives for obtaining high adhesion forces to metal foils.
U.S. Pat. No. 4,939,039 to Watanabe teaches the synthesis of polyimides having low thermal expansion coefficient for direct coating and curing on metal conductors.
U.S. Pat. No. 4,931,310 to Anschel, et al. discloses a technique for treating the surface of an intractable polyimide to form polyamic acid, which is then rapidly converted to polyimides by IR radiation. No techniques for lamination is disclosed nor is any technique for selectively imidizing thermoplastic polyamic acid.
IBM Technical Disclosure Bulletin, Vol. 31, No. 11, Apr. 1989, pp. 32-33 discloses a technique for imidizing an intractable polyimide to prevent the formation of skin by IR radiation. No lamination technique is disclosed nor any selective imidization of thermoplastic polyimides.
Whatever technique or materials are used in forming a lamination between a polyimide material and a metal foil, it is necessary to have a solid continuous high strength adherent bond between the polyimide material and the metal laminates so as to provide the necessary structural integrity to any parts formed therefrom thus assuring that the parts will not delaminate. Structural integrity is especially critical in certain applications where the laminated structure serves both as a current carrying member and also as a structural member.
A conventional prior technique for bonding laminates such as those described in certain of the above cited patents includes applying a first film of thermosetting or intractable polyimide precursor to a metal substrate and thermally imidizing the interactable polyimide. A second layer of thermoplastic polyimide precursor is coated over the intractable polyimide and thermally imidized. The dual layer polyimide on the metal substrate is then laminated to a metal foil such as copper. It has been found, however, that the use of conventional oven curing (thermal) techniques and lamination processes produce laminates with insufficient bond strength between the laminated metal and the thermoplastic polyimide. The reasons for such low bond strength and poor lamination properties are not completely understood. However, in the prior art it was believed due, at least in part, to gas being trapped between the thermoplastic polyimide and the sheet metal being laminated thereto thus preventing good adhesion, and in some instances, resulting in significantly large (i.e. macro) areas of interface which are not bonded at all due to entrapped gases.